The following discussion deals with starter tubes and the gas delivery system for optical fiber pre-forms, but it is to be understood that principles of the present invention are applicable to other, different applications involving, generally, chemical delivery systems wherein the chemicals are in gaseous or vapor form.
Optical fiber of the type used to carry optical signals is fabricated typically by heating and drawing a portion of an optical pre-form comprising a refractive core surrounded by a protective glass cladding. Presently, there are several known processes for fabricating pre-forms. The modified chemical vapor deposition (MCVD) process, which is described in U.S. Pat. No. 4,217,027 issued in the names of J. B. MacChesney et al. on Aug. 12, 1980 and assigned to Bell Telephone Laboratories, Inc. has been found to be one of the most useful because the process enables large scale production of pre-forms which yield very low loss optical fiber.
During the fabrication of pre-forms by the MCVD process, reactant-containing gases, such as SiCL.sub.4 are passed into a rotating substrate or starter tube which is made of silica glass. A torch heats the tube from the outside as the precursor gases are introduced therein, causing deposition of submicron-sized glass particles or soot on the inside surface of the tube. The torch is moved along the longitudinal axis of the tube in a plurality of passes to build up layer upon layer of soot to provide a pre-form tube. Once a sufficient number of layers have been deposited, the pre-form tube is then heated to cause it to be collapsed to yield a pre-form or pre-form rod as it is often called. The delivery system of the reactant gases to the starter tube interior is generally through a rotating or fixed metallic hollow tube connected to the source or sources of the gases.
In the current method of manufacture, the apparatus which ensures sealed delivery of the deposition chemicals in the gases is a combination of a rotary union element, a structure for holding and sealing the starter tube, and a secondary face seal assembly for routing of purge gases through the structure. This is a complex apparatus that requires frequent maintenance. Existing systems also have the disadvantage of having inherently larger cavities for the accumulation of dead zones of flow, and a tendency to create particle contamination from the rotary union and face seal system. Inasmuch as the chemical delivery system supply is stationary, the current means of achieving delivery is via the rotary union, featuring a transition of the chemicals from a stationary pipe to a rotary pipe or to the inside of a supply coupling. The chemicals being delivered are at a pressure greater than atmospheric, and the face seal properties are the only restriction to the release of the chemicals to the atmosphere. The rotary union and secondary face seals generate a large quantity of particles from wear, and contribute to the contamination of the coupling. The complexity of the components involved requires skilled maintenance being performed using requalification through test of the system. Both material and labor costs are, consequently, high.
In Mueller patent application Ser. No. 09/383,716, there is shown a sealing system that eliminates many drawbacks characteristic of prior art delivery systems, as enumerated in that application, such as, for example, the rotary union, by internally sealing the starter tube by means of a self tightening seal and mounting arrangement therefor. The basis of the arrangement of that application makes use of a constant rotational capability of the seal mounting hub for the self tightening feature.
In all such systems, it is generally the case that the chemical delivery tube is plugged at its distal end which protrudes into the starter tube, and ports are formed in the tube behind the plug, for example, two ports one hundred and eighty degrees apart, for allowing the gas to enter the starter tube interior toward the interior walls thereof, thus creating a radial nozzle. Such an arrangement, which is in widespread use, has the inherent disadvantage of having small port orifices through which all chemicals must be delivered to the starter tube. The ports act as orifice points with the inherent possibility of creating gas expansion problems, such as condensation and pressure drop related issues. The arrangement is non-self purging and does not allow for a complete unobstructed flow of products out of the delivery tube. Further, the plugging of the end of the delivery tube creates a dead zone or eddy volume between the plug and the orifices, where chemicals may become trapped or may pool. Potential contamination in the area (or volume) may build up in the absence of any means of self purging. As a consequence, frequent maintenance of the nozzle end of the delivery tube is necessary. In addition, gas flow exiting the nozzle is non-laminar, and, hence, does not guarantee a uniformity of coating of the interior wall of the starter tube, which is highly desirable.